Lower back pain affects over 65 million people in the United States with an estimated 12 million of these cases arising from degenerative disk disease. The back is particularly susceptible to damage and disease due to its complex structure. The spine is a complex structure of articulated bone and cartilage comprised of a column of vertebrae separated by vertebral disks (FIG. 1). These vertebral disks act as an intervening cushion to mitigate and distribute loads transferred along the spinal column.
The anisotropic structure of the intervertebral disk efficiently achieves the appropriate mechanical properties required to cushion complex spinal loads. The inner viscoelastic material, termed the nucleus pulposus, occupies 20-40% of the total disk cross-sectional area. The nucleus usually contains between 70-90% water by weight. The nucleus is composed of hydrophilic proteoglycans that attract water into the nucleus and thus generate an osmotic swelling pressure ranging between 0.1-0.3 MPa, which supports the compressive load on the spine. The nucleus is constrained laterally by a highly structured outer collagen layer, termed the annulus fibrosus (FIG. 1). The nucleus pulposus is always in compression, while the annulus fibrosus is always in tension. Although it comprises only one third of the total area of the disk cross-section, the nucleus supports 70% of the total load exerted on the disk. The intervertebral disk becomes less elastic with age, reaching the elasticity of hard rubber in most middle-aged adults as the nucleus loses water content. This water loss will also cause the disk to shrink in size and will compromise its properties.
In degenerative disk disease, the nucleus pulposus can become distorted under stress, resulting in the extrusion of part of the pulposus out through the annulus fibrosus, causing pressure against the surrounding nerves. This process is called herniation. The damage to the disk can often be irreversible if part of the pulposus is lost. The majority of disk injuries occur in the lumbar region, and the most common area of disease occurs at L4/L5 and L5/S1.
A laminectomy (surgical removal of part of a herniated disk—typically the nucleus pulposus) may be performed to relieve pressure on local neural tissue. This approach is clearly a short-term solution, given that the load bearing ability of the nucleus would be reduced with loss of material. Despite this, over 200,000 laminectomies are performed each year, with a success rate of 70-80%.
Arthrodesis or fusion is a more permanent method for surgically treating degenerative disk disease. Fusion is accomplished with or without internal fixation. While internal fixation has become increasingly popular, this technique has its share of complications. Fracture, neurological damage, and osteoporosis have been observed in patients who have undergone internal fixation fusions. The ability of the bone to fuse will vary from patient to patient, with the average likelihood of success ranging from 75-80%. Spinal fusion will cause stiffness and decreased motion of the spine. Additionally, fusion can also put stress on adjacent vertebrae in the spine, which can accelerate disease in adjacent disks and lead to additional back surgery.
A successfully designed artificial disk would replace a worn out disk while protecting patients from incurring problems at an adjacent level of the spine. Several artificial disk prostheses have been proposed in the prior art. Many of these prosthesis attempt complete replacement of the disk, including the nucleus and the annulus fibrosus. Given that the intervertebral disk is a complex joint with multi-directional loading, the design of a prosthesis that mimics the articulation and mechanical behavior of a natural disk is extraordinarily difficult. For example, when the body is supine, compressive loading on the third lumbar disk is 300 N, rising to 700 N in an upright stance, then to 1200 N when bending forward by 20°. Additionally, moments of 6 N-m are often achieved during flexion and extension, with up to 5° of rotation. For adequate safety, a preferred compressive strength of the entire disk is 4 MN/m2.
The most extensive experience to date with a complete disk replacement is that obtained with the SB Charité III prosthesis. This prosthesis has been used extensively in Europe since 1987, and has been implanted into over 3,000 patients. The SB III is designed with a polyethylene spacer placed between two cobalt chromium endplates. Two year follow-up studies have shown good clinical success in patients. Another study concerned a complete disk prosthesis consisting of a polyolefin core reinforced with carbon black, which is attached to two titanium plates. Preliminary results are not promising, since the core fractured in 2 of the implants.
Both of the examples presented above serve to indicate that there is considerable commercial effort being expended in the development of artificial disks. However in both cases the mechanical equivalence of these components to the human intervertebral disks is somewhat doubtful and the long-term clinical prognosis is still unclear.
As an alternative to the complete replacement of intervertebral disks, the nucleus pulposus alone can be replaced, leaving the annulus fibrosus intact. This approach is advantageous if the fibrosis is intact, in that it is less invasive, and the annulus can be restored to its natural fiber length and fiber tension. In replacing the nucleus, it is desirable to find a material that is similar in properties to the natural nucleus. The prior art describes bladders filled with air, saline, or a thixotropic gel. To prevent leakage, the membrane material comprising the bladder must be impermeable, which inhibits the natural diffusion of body fluid into the disk cavity, preventing access to necessary nutrients.
To generate a more natural disk replacement material, several research groups have investigated polymeric hydrogels as a possible replacement for the nucleus pulposus. Hydrogels are good analogs for the nucleus pulposus, in that they typically possess good viscoelastic properties and can offer good mechanical behavior. Additionally, they contain a large amount of free water, which permits a prosthesis made from a hydrogel to creep under compression and handle the cyclical loading without loss of elasticity, similar to a natural nucleus pulposus. The water permeability of these materials also allows diffusion of body fluid and nutrients into the disk space. Control of this pore structure, and the consequent control of the nutrient access to all parts of the implant, may be critical for future prosthetic implants.
Others have investigated the use of polyacrylonitrile-polyacrylamide multiblock copolymers encased in a jacket made from ultra high molecular weight polyethylene fibers. These systems absorb up to 80% of their weight in water. Polyvinyl alcohol (PVA) and copolymers of PVA and poly vinyl pyrrolidone (PVP) have produced prostheses with mechanical properties similar to natural disks. These materials have the additional advantage of having clinical success in other medical devices. Gels formed from PVA are usually prepared via a freeze-thaw process or via external crosslinking agents. In addition, hydrogel-based nuclei can contain therapeutic drugs which slowly diffuse out after implantation. Although no clinical data is currently available for these materials, biomechanical testing on cadaver joints has shown similar mechanical properties to natural disks.